This invention relates to transparent conductive coatings and methods therefor.
Thin coatings of transparent conductive materials (hereinafter TC materials) deposited on a non-conductive substrate have numerous applications ("conductive", "conductivity", etc., understood to refer to electrical conductivity). An important application is as a transparent electrode in a liquid crystal, touch-sensitive, or other visual display. Common TC materials include indium oxide and indium tin oxide (ITO). ITO is commonly used with tin as the minor component, in indium/tin weight ratios such as 95/5, 90/10 or 80/20.
If a TC material of constant resistivity is deposited (resistivity being dependent on the deposition method and conditions), the resistance of a coating of the TC material is approximately inversely proportional to the coating thickness, according to equation (I): EQU Resistance=(Resistivity)/(Thickness) (I)
In practice, for a thin coating, by which is meant a coating less than a few hundred Angstroms thick, there can be deviations from this relationship, because of surface effects at both the coating/substrate and coating/air interfaces which affect its resistance. As a coating of TC material is made thinner and thinner, its resistance increases, both due to the decreased thickness and to the magnified relative importance of surface effects.
Conversely, if a TC coating is made thicker in order to obtain decreased resistance, its light transmittance decreases, due to two effects. Firstly, there is the absorption inherently present in any material, even in a nominally transparent material such as ITO. ITO has significant absorption of blue light, as its "band-gap" is at about 3.5 eV. This absorption can be particularly high when ITO is deposited by a low temperature process, as is necessary when coating films made of polymers such as poly(ethylene terephthalate) (PET) or other polyester.
Secondly, optical interference effects become pronounced as thickness increases, becoming noticeable for ITO coatings at thicknesses greater than several hundred Angstroms. When a material with a relatively high refractive index such as ITO is deposited onto a substrate with a lower refractive index such as PET, optical interference will occur between the reflections from the coating/air and coating/substrate interfaces, according to equation (II): EQU nd=ml/4 (II)
where n is the refractive index of the coating, d is the coating thickness, l is the wavelength of light, and m is an integer. This interference results in a reflectance maximum (and a transmission minimum) when m is an odd integer (1,3,5 . . . ) and a reflectance minimum (and transmission maximum) when m is an even integer (2,4,6 . . . ). If the refractive index of the coating is less than that of the substrate, the selection rules are reversed--that is, reflectance minima occur when m is an odd integer and maxima occur when m is an even integer. As the thickness changes, the wavelengths of the minima and maxima change and hence the optical characteristics of the coating. For example, for a coating of ITO (refractive index about 2.0) 150 nm thick on PET (refractive index about 1.65), there is a visible wavelength transmission maximum at 600 nm (m=2, the maxima corresponding to other m's being outside the visible spectrum (400-700 nm)) and a minimum at 400 nm (m=3, the other minima being outside the visible spectrum). If the thickness is increased to 170 nm, the visible spectrum maximum and minimum shift to 680 and 453 nm, respectively. As a result, the visible light transmittance (VLT, the light transmittance averaged over 400-700 nm and weighted to the human eye's response) of ITO coatings on PET vary with thickness.
The aforementioned effects are particularly noticeable with coatings which have resistances of about 100 to about 200 ohm/sq. These coatings are thick enough (typically about 50-100 nm, depending on the deposition technique) to have significant absorption of blue light and are also thick enough to have an interference transmission minimum at or near the blue part of the spectrum. Consequently they appear unpleasantly brown in transmission to the human eye. These effects are exacerbated by absorption or scattering in the substrate, which is usually most pronounced for blue light.
Additionally, although high-resistance and high VLT coatings can be obtained by making the coating thinner, such coatings are very fragile and prone to several problems. They have poor abrasion resistance. They are difficult to deposit with uniform properties as the state of the substrate plays a considerable role in determining the quality of a coating, a non-uniform substrate producing a coating with non-uniform properties. Thin coatings impede water vapor and other gas permeation only slightly, whereas thicker coatings can be quite effective as vapor transmission barriers. The barrier properties of such films are discussed in Stern, GB No. 2,132,229 A (1984). As a coating is deposited on the substrate, the first several atomic layers do not nucleate evenly on it, causing poor and non-uniform properties in these first atomic layers. The top 3-5 nm of a coating will passivate in the atmosphere, forming a non-conducting oxide. If the thickness of the first atomic layers or the passivation layer is significant compared to the total coating thickness, then it will dramatically affect the overall properties of the coating.
In some applications, it is only required that the conductivity of a coating be less than a particular value, in which case the coating can simply be made sufficiently thick to lower the resistance below that value and to shift interference effects away from the wavelengths where an undesirable color effect is produced. However, in many other applications, a coating having specified resistance and optical or color characteristics is required. Then, the coating cannot be simply made thicker, as the resistance will then become unacceptably low. Conversely, at the thickness at which the desired resistance is obtained, the interference effects may be such as to make the coating's optical or color characteristics unacceptable.
It is known that the resistivity of TC materials such as ITO increases as the amount of tin present increases. See, e.g., Howson et al, Proc. SPIE 428, 14 (1983) and Chopra et al., Thin Solid Films 102, 1 (1983). This characteristic can be exploited to produce a high tin content ITO coating with a given resistance which is thicker than a lower tin content ITO coating of the same resistance. The thickness can be selected so as to shift the interference effects to a different wavelength and produce a coating with the desired optical or color characteristics. The disadvantage of this technique is that if a number of coatings is desired, all with the same thicknesses but different resistances, then the indium to tin ratio in the ITO must be varied for each coating. If the coatings are produced by a vacuum deposition technique such as sputtering, this is difficult to do, as a different sputtering target must be fabricated for every composition and resistance value.
Our invention provides a transparent conductive coating whose conductivity can be conveniently controlled but yet without compromising its optical properties.